首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   627篇
  免费   9篇
  国内免费   1篇
化学   406篇
晶体学   8篇
力学   8篇
数学   49篇
物理学   166篇
  2021年   6篇
  2019年   6篇
  2017年   6篇
  2016年   6篇
  2015年   10篇
  2014年   13篇
  2013年   28篇
  2012年   41篇
  2011年   41篇
  2010年   17篇
  2009年   13篇
  2008年   25篇
  2007年   29篇
  2006年   26篇
  2005年   22篇
  2004年   11篇
  2003年   9篇
  2002年   14篇
  2001年   9篇
  2000年   5篇
  1999年   5篇
  1998年   5篇
  1997年   7篇
  1996年   11篇
  1995年   9篇
  1994年   11篇
  1993年   9篇
  1992年   10篇
  1991年   14篇
  1990年   4篇
  1989年   5篇
  1988年   7篇
  1987年   5篇
  1986年   8篇
  1984年   10篇
  1983年   7篇
  1982年   11篇
  1981年   7篇
  1980年   14篇
  1979年   10篇
  1978年   8篇
  1977年   12篇
  1976年   6篇
  1975年   4篇
  1974年   9篇
  1973年   8篇
  1971年   5篇
  1967年   5篇
  1938年   4篇
  1911年   4篇
排序方式: 共有637条查询结果,搜索用时 62 毫秒
611.
Reaction of (C5Me5)2Lu(Me)(μ-Me)Li(THF)3 (2) with excess 12-crown-4 affords the new separated ion pair complex, [Li(12-crown-4)2][(C5Me5)2LuMe2] (3), in excellent yield. This complex reacts with 2,6-diisopropylaniline and phenylacetylene to give the methyl amide complex [Li(12-crown-4)2][(C5Me5)2Lu(Me)(NH-2,6-iPr2C6H3)] (4) and the bis(acetylide) complex [Li(12-crown-4)2][(C5Me5)2Lu(C≡C-Ph)2] (5), respectively. Attempts to promote methane loss from complexes 3 and 4 to generate a lutetium methylidene or imido complex, respectively, were unsuccessful. The ability of the bis(acetylide) complex 5 to act as a π-tweezer complex was also explored. Reaction between [Li(12-crown-4)2][(C5Me5)2Lu(C≡C-Ph)2] (5) and CuSPh gave only intractable lutetium products and the copper(I) species [Li(12-crown-4)2][Cu(C≡C-Ph)2] (8). The new lutetium complexes have been characterized by elemental analysis and NMR spectroscopy. Finally, the X-ray crystal structures of (C5Me5)2Lu(Me)(μ-Me)Li(THF)3 (2), [Li(12-crown-4)2][(C5Me5)2LuMe2] (3), [Li(12-crown-4)2][(C5Me5)2Lu(Me)(NH-2,6-iPr2C6H3)] (4), [Li(12-crown-4)2][(C5Me5)2Lu(C≡C-Ph)2] (5), and [Li(12-crown-4)2][Cu(C≡C-Ph)2] (8) are also reported.  相似文献   
612.
Laser-induced incandescence (LII) is an optical measurement technique capable of measuring soot volume fraction over a wide range of conditions. However, development of two-dimensional auto-compensating LII (2D-AC-LII) in the literature has been limited and until now, instantaneous measurements have not been demonstrated. In this paper, we successfully demonstrate instantaneous 2D-AC-LII soot volume fraction (SVF) measurements in an ethylene-air co-annular diffusion flame. Results were then used to support a detailed uncertainty analysis based on a Monte-Carlo simulation. Agreement between both the instantaneous and average SVF measurements with published data from attenuation measurements under identical conditions was found to be good. Uncertainties are discussed both in terms of an overall accuracy of the SVF measurement, which is strongly dominated by uncertainty in the optical properties of soot, and the comparative uncertainties with optical properties fixed. The uncertainty in an instantaneous 2D determination of SVF for a comparative measurement is dominated by photon shot noise, and in regions of high soot volume fraction it is below 25% (95% confidence interval). Shot noise uncertainty could be further reduced with additional pixel averaging at the expense of spatial resolution. This diagnostic shows significant promise for quantitative planar soot concentration measurements within turbulent flames.  相似文献   
613.
The primary literature concerning the combinatorial synthesis of organophosphorus compounds is reviewed and discussed. The subject matter is divided into three main sections describing the solid phase, solution phase and solvent-free synthesis of phosphorus containing organic molecules. The review covers the synthesis of compounds in which the final products contain phosphorus-carbon bonds, primarily phosphonates, phosphinates, phosphine oxides and phosphines.  相似文献   
614.
The highly diastereoselective anti-aminohydroxylation of (E)-gamma-tri-iso-propylsilyloxy-alpha,beta-unsaturated esters, via conjugate addition of lithium (S)-N-benzyl-N-(alpha-methylbenzyl)amide and subsequent in situ enolate oxidation with (+)-(camphorsulfonyl)oxaziridine, has been used as the key step in the asymmetric synthesis of N,O,O,O-tetra-acetyl d-lyxo-phytosphingosine (20% yield over 7 steps), the anhydrophytosphingosine jaspine B (10% yield over 9 steps), 2-epi-jaspine B (14% yield over 9 steps), and the Prosopis alkaloid deoxoprosophylline (26% yield over 7 steps).  相似文献   
615.
Treatment of 3-aminocyclohex-1-enes with mCPBA in the presence of trichloroacetic acid gives the corresponding 1,2-anti-2,3-syn-1-trichloroacetoxy-2-hydroxy-3-aminocyclohexane with high levels of diastereoselectivity (90% de). This is consistent with a mechanism of oxidation involving hydrogen-bonded delivery of the oxidant by the allylic ammonium ion formed in situ, followed by highly regioselective ring-opening of the intermediate epoxide by trichloroacetic acid. The effect of conformational constraints upon the oxidation reaction is also examined.  相似文献   
616.
A one-pot synthesis of allenes by the 2-nitrobenzenesulfonylhydrazide-mediated coupling of hydroxyaldehydes or ketones with alkynyl trifluoroborate salts is reported. This mild process involves in situ formation of a sulfonylhydrazone that reacts with alkynyl trifluoroborates to generate a transient propargylic hydrazide species. Decomposition of this unstable hydrazide via an intermediate monoalkyldiazine produces the allene products through an alkene walk mechanism.  相似文献   
617.
618.
"Matching" and "mismatching" effects in the doubly diastereoselective conjugate additions of the antipodes of lithium N-benzyl-(N-α-methylbenzyl)amide to enantiopure cis- and trans-dioxolane containing α,β-unsaturated esters have been investigated. High levels of substrate control were established first upon conjugate addition of achiral lithium N-benzyl-N-isopropylamide to both tert-butyl (S,S,E)-4,5-O-isopropylidene-4,5-dihydroxyhex-2-enoate and tert-butyl (4R,5S,E)-4,5-O-isopropylidene-4,5-dihydroxyhex-2-enoate. However, upon conjugate addition of lithium (R)-N-benzyl-(N-α-methylbenzyl)amide and lithium (S)-N-benzyl-(N-α-methylbenzyl)amide to these substrates, neither reaction pairing reinforced the apparent sense of substrate control. These reactions do not, therefore, conform to the classical doubly diastereoselective "matching" or "mismatching" pattern usually exhibited by this class of reaction. A comparison of these reactions with the previously reported doubly diastereoselective conjugate addition reactions of lithium amide reagents to analogous substrates is also discussed.  相似文献   
619.
Some 1,2-diarylimidoyl radicals were generated in the gas-phase by intramolecular radical translocation from ortho-imino-aryloxyl radicals, in turn generated under flash vacuum pyrolysis (FVP) conditions. The imidoyls reacted with XR ortho'-substituents in the N-aryl group to give (in most cases) modest yields of cyclisation products. Depending on the nature of the bridging atom (X), the formation of these products was initiated either by a further hydrogen atom translocation (X = CH(2)), or by ipso-attack onto the aryl group (R = Ph), or by direct substitution at the heteroatom (X = S). With XR = N(Me)Ph, the major reaction product was probably the result of a competing pathway not involving the corresponding imidoyl.  相似文献   
620.
The highly diastereoselective conjugate additions of the novel lithium amide reagents lithium (R)-N-(3-chloropropyl)-N-(α-methylbenzyl)amide and lithium (R)-N-(3-chloropropyl)-N-(α-methyl-p-methoxybenzyl)amide to α,β-unsaturated esters were used as the key steps in syntheses of the homalium alkaloids (-)-(S,S)-homaline and (-)-(R,R)-hopromine. The asymmetric synthesis of (-)-(S,S)-homaline was achieved in 8 steps and 18% overall yield, and the asymmetric synthesis of (-)-(R,R)-hopromine was achieved in 9 steps and 23% overall yield, from commercially available starting materials in each case. These syntheses therefore represent by far the most efficient total asymmetric syntheses of these alkaloids reported to date. A sample of the (4'R,4'S)-epimer of hopromine was also produced using this approach, which provided the first unambiguous confirmation of its absolute configuration and therefore that of natural (-)-(R,R)-hopromine.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号